Advanced Ni-base superalloys are currently isothermally forged at relatively slow strain rates and temperatures below their .gamma.' solvus temperatures. Forging is typically followed by supersolvus annealing. This method tends to minimize forging loads and die stresses, and avoids fracturing the items being formed during forging operations. It also permits superplastic deformation of these alloys in order to minimize retained metallurgical strain at the conclusion of the forming operations. However, this method can have substantial limitations with respect to forming substantially uniform fine grain size articles. While the method tends to produce relatively fine-grain as-forged microstructures having an average grain size on the order of about 7 .mu.m, subsequent supersolvus annealing causes the grain size to increase to about 20-30 .mu.m. Also, unless the forging process is carefully controlled so as to avoid imparting retained strain into the forged articles, this method can produce articles that are subject to the problem of critical grain growth, wherein the retained strain energy in the article is sufficient to cause limited nucleation and substantial growth (in regions containing the retained strain) of very large grains upon subsequent supersolvus annealing. Critical grain growth can cause the formation of gains as large as 300-3000 .mu.m.
It is desirable to form uniform large gain microstructures in these Ni-base superalloys to enhance their high temperature creep and crack propagation resistance, such as microstructures having an average gain size in the range of 90-120 microns. Controlled large gain sizes are known to be difficult, if not impossible, to produce using isothermal forging. Isothermal subsolvus forging is also known to require very careful process control in order to avoid imparting low levels of retained strain energy to the forged parts that can result in critical gain growth upon subsequent annealing of the forging. It is also very desirable to avoid the problem of critical gain growth.
Therefore, new methods of forging are desirable to produce forged articles which avoid critical grain growth, as well as methods that would enable the development of uniform large gain microstructures.